(Supported in part by a CNRS grant to M. Bornens and by NIH GMS 40198 to C. Rieder). Glutamylation represents the major post-translational modification of brain tubulin. This modification is also detected in non-neuronal cell lines where it appears restricted to the centrioles. Using a monoclonal antibody directed against this modification (GT335) Dr. Bornens's conducted microinjection and electro permeabilization experiments in order to specifically disrupt centrioles in vivo. They found that they could no longer detect centrosomes or centrioles, using fluorescent markers, 24 hours after loading HeLa cells with GT335. Short-term microtubule re-growth experiments confirmed the loss of a functional centrosome. Centrosome disappearance took place within the first 122 hrs, and was maintained during 48 hrs, but was reversible. During the acentrosomal period cells were still able to progress through mitosis. The HVEM and IVEM were used to confirm the results obtained by u sing LM markers-that the cells indeed lacked centrioles up to 48 hrs after loading. In addition, IVEM tomography was used to examine fragmented centrioles in the process of being destroyed by the antibody. This work is currently being prepared for publication and it reveals that the centriole acts as a organizer of the centrosome and that the polyglutamate residues of the centriolar microtubule triplets are involved in centriole stability.